1. Field of the Invention
The invention pertains to electrically-controlled devices for switching optical signals and more particularly to electrically-controlled devices employing optically active nematic liquid crystals for switching polarized or unpolarized optical signals with low insertion loss.
2. Description of the Prior Art
Fiber optic transmission systems have found favor, particularly for linking distributed computers and computer-controlled industrial system components, because of their high information transmission capacity, immunity from electro-magnetic interference, and security from unauthorized interception. Such optical transmission systems may utilize switching devices for control purposes or for transferring data from one link to another. One favored approach utilizes a single-fiber, multi-mode optical transmission line. In such a line, the light output is characteristically unpolarized. Nematic liquid crystal electro-optic elements of prior art optical switches providing high crosstalk rejection ratios have been shown to be capable of switching only one component representing a single polarization of the incident light, with any given liquid crystal composition. (Unpolarized light may be represented by a vector sum of two orthogonally polarized light beam components). In consequence, the non-favored orthogonal polarization component has been heretofore discarded, resulting in relatively high insertion loss of the order of one-half the available optical power in the prior art switches.
A representative switch of the prior art employing liquid crystal elements is shown in U.S. Pat. No. 4,278,327, issued July 14, 1981 to Donald H. McMahon and Richard A. Soref and assigned to the Assignee of the present invention. That disclosure has an extensive discussion of prior art deficiencies and points out in particular that electro-optic devices as described therein are seriously polarization sensitive. A typical prior art optical switch as in FIG. 3 therein is shown to switch not more than 50% of the power of an input unpolarized light beam, which switch, by virtue of its inherent inability to handle orthogonal light polarizations, introduces a 3 db loss as well as input and output coupling losses between the terminals of a link. Such a loss could become particularly serious when additional such devices are used in series interconnections. McMahon and Soref show that by appropriately manipulating the polarization states of the light beam components entering the device, a polarization-independent liquid crystal switch may be devised. These prior art devices, however, utilize the polarizing properties of the liquid crystal media to separate incoming unpolarized light into orthogonal linearly polarized components. Because a liquid crystal polarizer does not reflect and transmit equal percentages of light for the components parallel to and normal to the plane of incidence and permits some random polarization to occur, it is somewhat less than an ideal polarizing device. In consequence, undesired light beam components propagated within the switch result in cross-talk, limiting the isolation to a factor of the order of 12 db, which represents approximately a 16:1 power ratio between the desired and undesired signals. This level of isolation is insufficient for many critical applications. A further disadvantage of the prior art, as shown in FIG. 12 of McMahon and Soref, is the relative complexity of the structure required for switching between a multiplicity of output terminals, it being noted that successive layers of crystal prisms and liquid crystal materials are required for all but the most elementary switch.
A second prior art device is disclosed by McMahon in U.S. patent application Ser. No. 362,563, U.S. Pat. No. 4,461,543, Electro-Optic Switch, also assigned to the Assignee of the present invention. That invention provides a combination of birefringent crystal prism and liquid crystals operating in the nematic twist mode to polarize, switch, and recombine the polarized components into a single light beam of arbitrary polarization. While providing improved performance in the way of low insertion loss and improved optical isolation between input and output terminals, it again requires a relatively complex structure which is not amendable to miniaturization and relatively low cost construction. In the 1.times.N embodiment shown therein a plurality of nematic liquid crystal-birefringent device combinations arranged in tandem are required, the number of output terminals being doubled for each liquid crystal-birefringent crystal combination addition.
It is, therefore, an object of the present invention to provide an optical switch for routing light from a single multi-mode optical fiber input to one or more multimode fibers outputs in response to electrical command signals, and to provide a reciprocal optical switch which may accommodate multiple inputs and combine them to provide a single output, while minimizing crosstalk, optical insertion losses, size, and construction complexity and cost.